iter 22b.1.1: ClassDef/InstanceDef AST + downstream match arms

Adds Def::Class(ClassDef) and Def::Instance(InstanceDef) variants per
Decision 11 §"Form-A schema". All optional fields (superclass, doc,
default body) carry skip_serializing_if so canonical-JSON bytes of
pre-22b fixtures stay bit-identical.

Downstream match-on-Def sites are extended with explicit Class/Instance
arms. Behaviour for 22b.1 is placeholder (skip in typecheck/codegen,
one-line summary in pretty/manifest, placeholder marker in prose/print)
with TODO comments naming the deferred sub-iters (22b.2 typecheck,
22b.3 codegen, 22b.4 prose-projection arms).

crates/ailang-core/src/ast.rs

@@ -57,9 +57,16 @@ pub struct Import {
 
 /// A top-level definition — the unit that gets a content hash.
 ///
-/// Discriminated by the JSON `kind` tag: `"fn"`, `"const"`, or
-/// `"type"`. Use [`def_name`] / [`def_kind`] (or the [`Def::name`]
-/// method) to inspect generically without matching every variant.
+/// Discriminated by the JSON `kind` tag: `"fn"`, `"const"`, `"type"`,
+/// `"class"`, or `"instance"`. Use [`def_name`] / [`def_kind`] (or the
+/// [`Def::name`] method) to inspect generically without matching every
+/// variant.
+///
+/// Iter 22b.1 (Decision 11): adds `Class` and `Instance`. Both are
+/// additive — pre-22b fixtures never produce these tags, so their
+/// canonical-JSON bytes (and therefore their content hashes) are
+/// unchanged. The `Def`-level match exhaustiveness in downstream
+/// crates is the only caller that has to acknowledge the variants.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(tag = "kind", rename_all = "lowercase")]
 pub enum Def {
@@ -69,15 +76,26 @@ pub enum Def {
     Const(ConstDef),
     /// Type (ADT) definition; see [`TypeDef`].
     Type(TypeDef),
+    /// Iter 22b.1: typeclass declaration; see [`ClassDef`].
+    Class(ClassDef),
+    /// Iter 22b.1: instance declaration; see [`InstanceDef`].
+    Instance(InstanceDef),
 }
 
 impl Def {
     /// Source-level name of the definition, regardless of kind.
+    ///
+    /// For `Def::Instance` the "name" is the class being instantiated;
+    /// instances are not separately named at the source level. The
+    /// workspace registry keys instances by `(class, type-hash)`, so
+    /// the class name alone is the closest analogue of a "name".
     pub fn name(&self) -> &str {
         match self {
             Def::Fn(f) => &f.name,
             Def::Const(c) => &c.name,
             Def::Type(t) => &t.name,
+            Def::Class(c) => &c.name,
+            Def::Instance(i) => &i.class,
         }
     }
 }
@@ -92,7 +110,7 @@ pub fn def_name(def: &Def) -> &str {
 }
 
 /// External helper: discriminator tag of a definition (`"fn"`,
-/// `"const"`, `"type"`).
+/// `"const"`, `"type"`, `"class"`, `"instance"`).
 ///
 /// Identical to the `kind` field in the JSON representation. Useful
 /// for tools that group or filter definitions by kind.
@@ -101,6 +119,8 @@ pub fn def_kind(def: &Def) -> &'static str {
         Def::Fn(_) => "fn",
         Def::Const(_) => "const",
         Def::Type(_) => "type",
+        Def::Class(_) => "class",
+        Def::Instance(_) => "instance",
     }
 }
 
@@ -220,6 +240,97 @@ pub struct Suppress {
     pub because: String,
 }
 
+/// Iter 22b.1: a typeclass declaration (Decision 11).
+///
+/// Single-parameter, multi-method, optional-default, optional-superclass
+/// typeclass. The `param` is a single string — multi-param classes are
+/// rejected by Decision 11 axis 1, and the schema enforces it by shape
+/// (`param: String`, not `Vec<String>`).
+///
+/// `superclass`, when present, MUST have its `type` field equal to
+/// `param`. The check is enforced in 22b.2 via the
+/// `InvalidSuperclassParam` diagnostic; the schema does not encode it.
+///
+/// All optional fields are omitted from canonical JSON when absent /
+/// empty, so future schema evolution can land here without disturbing
+/// pre-22b hashes.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ClassDef {
+    /// Class name (capitalised by convention).
+    pub name: String,
+    /// Single type-parameter name.
+    pub param: String,
+    /// Optional single-superclass relation.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub superclass: Option<SuperclassRef>,
+    /// Methods of the class, in declaration order.
+    pub methods: Vec<ClassMethod>,
+    /// Optional source-level documentation string.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub doc: Option<String>,
+}
+
+/// Iter 22b.1: reference to a superclass relation in [`ClassDef`].
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SuperclassRef {
+    /// Superclass name.
+    pub class: String,
+    /// Type the superclass is applied to. MUST equal the parent
+    /// `ClassDef.param` (validated in 22b.2 — schema does not enforce).
+    #[serde(rename = "type")]
+    pub type_: String,
+}
+
+/// Iter 22b.1: one method declared in a [`ClassDef`].
+///
+/// `default` is `None` when the method is abstract-required (every
+/// instance must specify it); `Some(body)` when the method has a
+/// default body (instances may inherit or override).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ClassMethod {
+    /// Method name.
+    pub name: String,
+    /// Full method signature including any mode annotations. The
+    /// class parameter appears as a [`Type::Var`] inside this signature.
+    #[serde(rename = "type")]
+    pub ty: Type,
+    /// Default body. `None` ⇒ abstract-required; `Some(body)` ⇒
+    /// default-with-fallback.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub default: Option<Term>,
+}
+
+/// Iter 22b.1: an instance declaration (Decision 11).
+///
+/// `class` is the name of the class being instantiated. `type_` is the
+/// concrete type expression the class is applied to — never the class
+/// param. `methods` contains bodies for the required methods plus any
+/// overrides of default-bearing methods.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct InstanceDef {
+    /// Class being instantiated.
+    pub class: String,
+    /// Concrete type the class is applied to.
+    #[serde(rename = "type")]
+    pub type_: Type,
+    /// Method bodies in declaration order.
+    pub methods: Vec<InstanceMethod>,
+    /// Optional source-level documentation string.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub doc: Option<String>,
+}
+
+/// Iter 22b.1: one method body in an [`InstanceDef`].
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct InstanceMethod {
+    /// Method name (must match a method in the corresponding class).
+    pub name: String,
+    /// Method body. The class's declared method type with the class
+    /// param substituted to the instance type is the body's expected
+    /// type — checked in 22b.2.
+    pub body: Term,
+}
+
 /// A constant (top-level binding to a value).
 ///
 /// Differs from a zero-arg [`FnDef`] in that it is evaluated once and